Concrete is a mixture of portland cement and aggregate. The aggregate is usually a mixture of rocks of different sizes and sand, and the selection of the sizes and character of the aggregate particles is important in determining the quality of the concrete. The cement is a material which, when mixed with water, sets to form a solid monolithic stone-like material. The cement also surrounds and binds the aggregate to form a unitary, hard, strong mass that is the familiar material known generally as concrete. The concrete is stronger than cement by itself and is well-known for its high compressive strength and even its high tensile strength when used in such forms as prestressed concrete.
Concrete is formed by a casting process. Generally, forms are employed to produce hollow cavities in the shape of the desired concrete element, and the forms are filled with moist, pasty concrete and maintained at least until the concrete has set to have green strength, after which the forms are removed and the concrete element remains. Concrete is used with exposed surfaces that are horizontal and facing upwardly, such as in roads and walks. Concrete surfaces are also exposed which are horizontal and facing downwardly, such as in ceilings or the underside of bridges or overpasses. Concrete is also used with vertical faces exposed, such as in walls, columns and foundations.
Although concrete is a mixture of aggregate and cement, the outer surfaces of concrete have the appearance of being just cement, and no aggregate is visible. Concrete surfaces are characteristically an uneven gray color and devoid of large features. Instead, concrete surfaces characteristically have ugly form marks which may be the joints between adjacent elements of the form or the grain of wood. Concrete structural elements are notoriously ugly.
One way to increase the interest and beauty of concrete elements is to remove the surface cement. Removing the surface cement removes mold marks and exposes the aggregate. Aggregate is a much more interesting texture than cement because it does not have a monotonous monolithic appearance and because the various stones that are exposed are in a variety of sizes and have natural and non-repetitive shapes.
To remove the surface layer of cement from a concrete element, certain materials which are known to retard the setting of cement have been applied to the inside of the forms. These materials retard the setting of cement only of the thin surface layer so that when the forms are removed, the thin surface layer can be eroded away to expose the aggregate beneath it. Removing this thin surface layer of cement does not affect the strength of the element. Among the materials that are known to retard the setting of cement are sugars and certain salts, such as aluminum or magnesium halides or aluminum and magnesium saccharates.
Artistic and aesthetic effects can be achieved by applying retardants in patterns so that exposed aggregate in specific patterns becomes a feature of the final concrete element. For example, walks or retaining walls having their exposed surfaces with geometric patterns or even individually produced representations or scenes can be in the form of exposed aggregate, whereas the rest of the wall is in the form of flat cement, may be employed as a desirable architectural feature that is superior to a plain concrete wall or even to a concrete wall, the surface of which is entirely exposed aggregate. However, obtaining sharply defined features in a geometric form or another pattern is difficult to obtain because the concrete setting retardants are water soluble, and when they are in contact with wet concrete during its setting period, they run along the interface between the form and the concrete to produce a blurred pattern or an indistinct outline of the pattern.